Last week the Conservative leader, David Cameron, said he'd freeze the licence fee if he got the chance, and that in hard times the BBC should "lead by example". Political opponents roundly condemned the one-year licence fee freeze. Andy Burnham, the culture secretary, called Cameron an "opportunist" and the Lib Dems branded the idea "crazy". All good news for the BBC.

But when it comes to Cameron's broader assertion - that as commercial broadcasters struggle, the system is in danger of becoming "skewed" and "unbalanced", with the BBC growing more dominant due to its secure funding - the situation is rather different.

The idea that there is an imbalance between the publicly funded BBC and commercial broadcasters is pretty widely supported. Traditionally, total licence fee revenues have been roughly equal to total television advertising revenue. But this year, licence fee income will be around £3.5bn, while total TV advertising will fall to between £2.5bn and £3bn. The BBC's income is guaranteed - free of commercial jeopardy.

So there is a certain seductive logic to suggestions that the system needs "rebalancing" - and other broadcasters have not been slow to seek advantage. Michael Grade, ITV's executive chairman, has held up the BBC's size and guaranteed income as further support for his campaign to secure regulatory relief, and Channel 4's chief executive, Andy Duncan, devoted most of a recent speech to the subject. The system was "out of kilter", he said, warning of an "increasingly dominant BBC" that "threatens to diminish British creativity". By giving more away - to C4 naturally - the BBC could help restore balance. And it could easily afford to, he said.

These arguments are, of course, to a large extent self-serving - and C4 has been seeking public cash for some time. But whatever happens to C4's revenues in the longer term, the channel is currently outperforming its commercial competitors by some margin - while the BBC is left trying to argue that it is not flush with cash for waging war on its weakened rivals.

Isn't it? Given the imbalance between advertising revenues and the licence fee, surely the BBC is substantially better off? Well, not necessarily. For a start, not all of the licence fee is spent on tele- vision - although it does get the biggest chunk of cash - so commercial television revenues have historically outstripped the BBC's television spending. Then there's the question of commercial television revenue beyond advertising - the money generated by cable and satellite subscriptions, for instance, is now greater than the cash brought in through ads. Even if the BBC was spending the whole licence fee on television, that would still be substantially less than the cash available to commercial television through advertising and subscription.

Last week the BBC's director general, Mark Thompson, went out of his way to talk up the potential of planned partnerships with ITV on regional news and new technology projects, and with C4 in terms of a possible deal with BBC Worldwide. At the same time he was at pains to say that another £400m of cost savings will be needed if the BBC is to avoid breaching its banking agreements.

The BBC is big - perhaps too big - and inefficient (although so are ITV and C4) but simply "rebalancing" the system at the BBC's expense isn't an appropriate response to the longer-term structural issues faced by commercial broadcasters. If the commercial side of our broadcasting ecology is as broken as many suggest, reducing the size and scale of the BBC to match would, some maintain, be doing serious damage to one bit of public broadcasting that still works.